1. Field of the Invention
The present invention relates generally to a mobile communication terminal, and more particularly, to a method and an apparatus for reducing the number of times of measurements for cell reselection.
2. Description of the Related Art
Generally, the 2nd generation (2G) mobile communication system providing voice-oriented service includes a Global System for Mobile Communications (GSM) and Interim Standard-95 (IS-95). The GSM has been commercialized in Europe in 1992 and provides service using Time Division Multiple Access (TDMA). IS-95 has been commercialized in Korea and the United States, and provides service using Code Division Multiple Access (CDMA).
Meanwhile, the 3rd Generation (3G) mobile communication system that has evolved from the 2G mobile communication system supports packet service as well as voice service using the CDMA system. The 3G mobile communication system includes a 3G Project Partnership (3GPP) or a wideband CDMA (WCDMA) system, which is an European and Japanese standard based on a synchronism between base stations, and 3GPP2 or a CDMA2000, which is an American standard based on synchronism between base stations. Here, the 3GPP proposes Frequency Division Duplexing (FDD) where upward/downward transmission and reception are discriminated using frequency in order to facilitate the efficient use of a limited channel, and Time Division Duplexing (TDD) where upward/downward transmission and reception are discriminated using time in order to facilitate the efficient use of a limited channel.
Meanwhile, one of the important characteristics of the 3G mobile communication system is to support global roaming service regardless of a communication standard or a frequency used in the 3G mobile communication system. Therefore, 3G mobile communication systems should support a handover to the 2G mobile communication systems or other 3G mobile communication systems. That is, in a mobile communication environment where different communication standards coexist as mobile communication systems by various communication standards are commercialized due to the development of communication technology, when a mobile terminal gets out of a current-serviced base station region and moves to another base station region that uses a different communication standard or a different frequency, the 3D mobile communication systems should support the handover between base stations supporting a different communication standard or a different frequency.
The handover includes an intra frequency handover which uses the same frequency and the same communication standard, an inter frequency handover between different frequencies, and an inter radio access technologies (RAT) handover.
First, the intra frequency handover is a handover between mobile communication systems that use the same frequency and the same communication standard. Referring to FIG. 1, a handover between cells 140 and 150 that use WCDMA standard and a frequency F1 corresponds to the intra frequency handover. Here, for the intra frequency handover, a mobile terminal performs an intra frequency measurement monitoring the state of a base station (a target base station) as an object for the handover.
The inter frequency handover is a handover between mobile communication systems that use different frequencies. In detail, the inter frequency handover means a handover for the case where base stations use different frequencies even though they belong to mobile communication systems using the same communication standard. Referring to FIG. 1, a handover between cell 110 that uses the WCDMA standard and a frequency F1 and cell 130 that uses the WCDMA standard and a frequency F2 corresponds to the inter frequency handover. Here, for the inter frequency handover, a mobile terminal performs an inter frequency measurement monitoring the state of a target base station.
Last, the inter-RAT handover is a handover between mobile communication systems that use different communication standards. Referring to FIG. 1, a handover between cell 110 that uses the WCDMA standard and a frequency F1 and cell 120 that uses a GSM standard corresponds to the inter-RAT handover. Here, for the inter-RAT handover, a mobile terminal performs an inter-RAT measurement monitoring a state of a target base station.
The handover is a technology that allows a system to measure the state of a terminal in communication and change the base station that will provide service to the terminal. During the handover, a mobile terminal that is in an idle state, not in communication, performs cell reselection. Here, cell reselection means a terminal normally camping on service migrates in search for a better cell according to a cell reselection reference. At this point, a value that can be used for the cell reselection reference may be a parameter value received from a network, and a mobile terminal determines the step to be actually performed according to the parameter value.
A conventional cell reselection process will be described below. A mobile terminal that is in an idle state wakes periodically to measure the signal receive level (Srxlev) and the signal quality (Squal) of a serving cell, and examines whether the measurement results satisfy a predetermined condition based on the parameter values. When the predetermined condition is satisfied, that is, a serving-cell selection condition is not satisfied or an adjacent-cell measurement condition is satisfied, the mobile terminal performs an adjacent-cell measurement for cell reselection, and determines ranking of adjacent cells using the measurement results. After that, when a highest ranking adjacent cell maintains a better state than the serving cell for a predetermined time, the mobile terminal migrates to the corresponding adjacent cell, thereby performing cell reselection. After that, when the mobile terminal normally camps on, the above process is repeated.
Here, the Srxlev and Squal of a serving cell that are measured by a mobile terminal can be measured using Received Signal Code Power (RSCP) or Signal-to-Noise Ratio (SNR). Generally, in the case of an FDD cell, the SNR value is used for a cell reselection reference. In the case of a TDD cell, the RSCP value is used for a cell reselection reference.
The serving-cell selection condition serves as a reference used for judging whether a serving cell is a selectable cell, that is, whether the serving cell satisfies a minimum condition required for providing service. The serving-cell selection condition has a different value depending on whether the serving cell is an FDD cell or a TDD cell. When the serving cell is an FDD cell, the serving-cell selection condition is Squal>0 and Srxlev>0. When the serving cell is a TDD cell, the serving-cell selection condition is Srxlev>0.
The parameter value received from the network includes data for adjacent cells of a serving cell in which the mobile terminal receives service. When the adjacent cells include a cell that uses the same frequency and the same communication standard as those of the serving cell, the data for the adjacent cells include Sintrasearch data for the intra frequency measurement. Also, when the adjacent cells include a cell of a frequency and a communication standard that are different from those of the serving cell, the data for the adjacent cells include Ssearch_rat-gsm data for the inter-RAT measurement. Also, when the adjacent cells include a cell of a frequency different from that of the serving cell, the data for the adjacent cells include Sintersearch data for the inter frequency measurement.
The adjacent-cell measurement condition based on the parameter value for cell reselection will be described below. In the case of the intra frequency measurement in the FDD cell, when Sintrasearch>Squal or the Sintrasearch data are not broadcasted periodically from the network, the mobile terminal performs the intra frequency measurement. In the case of the inter-RAT measurement, when Ssearch_rat-gsm>Squal or Ssearch_rat-gsm data are not broadcasted periodically from the network, the mobile terminal performs the inter frequency measurement.
Meanwhile, the conventional adjacent-cell measurement condition for cell reselection does not consider a pingpong phenomenon. That is, when a network broadcasts an inappropriate parameter due to an erroneous network design and values such as the Sintrasearch, Sintersearch, and Ssearch_rat_gsm are set excessively large compared to the Squal, or values such as the Sintrasearch, Sintersearch, and Ssearch_rat_gsm are not broadcasted, the mobile terminal performs an adjacent-cell measurement every time the mobile terminal wakes from an idle state. Accordingly, the mobile terminal that performs an adjacent-cell measurement consumes a large amount of quiescent current compared to a mobile terminal that does not perform the adjacent-cell measurement.